A common problem in many different fields is the need to know the height of a liquid within a tank. For example, where an air space is formed above the surface of liquid fuel present in the fuel tank of an automobile or airplane, knowledge of the shape of the tank and the height of the air-liquid interface from the tank bottom will allow one to calculate the amount of remaining fuel.
Where a plurality of stratifying liquids are present within a tank, it may furthermore be desired to know the height of each stratified liquid layer. For example, where water is mixed with hydrocarbon fuel intentionally, such as when seawater is used as ballast in oil tankers; or unintentionally, such as when water is present in a vehicle fuel tank or such as when groundwater seeps into tanks for fuel pumps at filling stations, it may be desired to know the height of fuel layer(s) as distinct from nonfuel layer(s) for accurate determination of remaining fuel.
Time domain reflectometry (TDR) applies radar techniques to transmission line theory to detect the location of impedance transitions or discontinuities at interfaces between different layers of materials. In TDR, an interrogation pulse transmitted from a transmitter is reflected from such an impedance discontinuity, and the reflected pulse is received by a receiver, the distance (range) to the impedance discontinuity that caused the reflection being calculatable from the observed round-trip propagation time.
However, there are number of technical problems that have limited the usefulness of TDR to measurement of liquid height within a tank. In particular, such technical problems have prevented accurate and reliable detection of interfaces between layers of comparatively similar dielectric constant, such as air and hydrocarbon fuel, or between layers of stratified immiscible liquids.
One such technical problem is the effect of relative permittivity (also referred to herein as “dielectric constant,” sometimes abbreviated as “dielectric”) on velocity of propagation. That is, the velocity of propagation of the interrogation pulse as it travels from the transmitter to the impedance discontinuity and back to the receiver will vary depending on the dielectric constant(s) of the medium or media through which the pulse is traveling in accordance with the relationship
      v    =          c              ɛ              ,where v=velocity of propagation, c=speed of light, and ∈=dielectric constant. Since dielectric constant varies depending on material, and since dielectric constant of many materials can be a strong function of density (and thus of temperature), and is often moreover a strong function of the amount of any additive or contaminant that may be present, the velocity of propagation of the traveling pulse is in general changing as it goes from one material to another, and the velocity of propagation in any given medium may moreover vary in correspondence to such things as additive content and temperature. The effect of temperature on dielectric constant is especially true of liquids, and the effect of additive content on dielectric constant is especially true of ethanol additive in hydrocarbon fuel.
Another such technical problem is the practical difficulty, using commercially available electronics, of measuring the impedance changes that occur along the length of the TDR probe to a spatial resolution that will provide an acceptable rendering of the height profile of the various layers in which the TDR probe is immersed within the tank. Resolving such impedance changes to a degree sufficient to accurately determine layer height will be especially difficult for interfaces between layers of comparatively similar dielectric constant. That is, whereas it may have conventionally been possible to resolve interfaces between layers of comparatively dissimilar dielectric constant, such as air and water, it has conventionally been difficult to accurately resolve interfaces between layers of comparatively similar dielectric constant, such as air and hydrocarbon fuel, or between layers of stratified immiscible liquids.
An order-of-magnitude analysis of typical distances and times involved will show that, for transmission line distances on the order of several meters, for example, the round-trip pulse propagation time might be on the order of several tens of nanoseconds, and spatially resolving the impedance changes along a probe of length on the order of several decimeters to an accuracy on the order of 1 millimeter would require ability to sample the reflected pulse at successive time increments on the order of 1 picosecond apart. Thus, although it might be possible to capture, in analog fashion after the fashion of an oscilloscope trace, the reflected waveform from a single interrogation pulse and observe such picosecond-order impedance variations, it would be impractical to design digital circuitry capable of capturing and processing samples spaced apart at such small time increments over the course of a single interrogation pulse cycle.
Instead, to achieve such sampling resolutions with affordable digital electronics, an aliasing sampling system may be employed as disclosed in commonly assigned U.S. nonprovisional patent application Ser. No. 12/243,511 entitled “System and Method for Accurately Measuring Fluid Level in a Tank.” In such an aliasing sampling system, the interrogation pulse is transmitted in cyclical fashion at a frequency on the order of 1 MHz, for example, and a swept phase shift or delay is introduced between the time that the interrogation pulse is transmitted and the time that a sample is captured. In this way, with times on the order of one microsecond between successive 1 MHz pulses, the electronics have a chance to recover between samples, and the desired resolution can be achieved by sweeping, or incrementally increasing with each successive cycle, the delay (phase shift) between pulse transmission and sample capture. The train of samples captured at times corresponding to the swept increase in phase shift can be said to exhibit aliasing, since the net effect of combining the constant interrogation pulse frequency with the swept sampling frequency is to produce a microsecond-order version of the picosecond-order waveform, the progressively delayed sampling occurring due to the swept phase shift appearing as a “beat frequency” that is slow enough to be captured and processed by the electronics.
However, even with such techniques, the limitations of economically available electronics still make it impractical conventionally to spatially resolve the height profile of the various layers in which the TDR probe is immersed. Continuing with the example given above, to achieve a spatial resolution on the order of 1 millimeter with a transmission line of length on the order of several meters would require on the order of several thousand samples. These several thousand samples might be collected over the course of several milliseconds if 1 sample were collected at every 1 MHz pulse and each such sample was usable by the digital signal processing electronics. However, to achieve an acceptable signal-to-noise ratio, it is found in practice that many raw samples must be averaged together by fast analog filtering electronics to create a single filtered sample that is then collected by the slower digital signal processing electronics. That is, sampling is actually occurring at two levels: raw samples are averaged together by filtering, and the filtered samples are then being collected by the digital electronics. Allowing time for filtering to be carried out between the samples collected by the digital electronics results in times on the order of 500 microseconds between successive filtered samples. This being the case, at 500 microseconds per sample, several thousand samples would take on the order of several seconds to collect. Such slow response times are unacceptable for many applications.
Furthermore, this problem of slow response time and inefficient use of processing capacity is made even worse in situations where there are significant lengths of cable or other such transmission line medium between digital signal processing electronics and TDR probe(s). This may be the case where the digital signal processing electronics are removed by some distance from the probe for safety or for convenience, or so that a single digital signal processing electronics unit can be used with multiple probes located various distances therefrom.
For this and other reasons, conventional commercial TDR systems have generally been unable to adequately resolve the impedance changes occurring along the length of the TDR probe with a spatial resolution and response time sufficient to obtain a reasonably accurate rendering of the height profile of the various layers in which the TDR probe is immersed within the tank. Several practical TDR systems have therefore been proposed to overcome these difficulties, but conventional TDR systems for measuring liquid level within a tank still suffer from problems such as inadequate resolution, slow response time, and sensitivity to variation in dielectric constant and/or temperature.
For example, as disclosed in commonly assigned U.S. nonprovisional patent application Ser. No. 12/243,511 entitled “System and Method for Accurately Measuring Fluid Level in a Tank,” a time-of-flight-based technique has been proposed where it has not been possible to reliably detect an air-liquid interface or liquid-liquid interface at the top or bottom of a liquid layer is to determine liquid layer height based not on locations of such interfaces but on the amount by which propagation of the interrogation pulse and its reflection from an easily detectable impedance transition (such as the bottom of an electrically open- or short-circuited TDR probe) is delayed as a result of passage through that liquid layer. However, because the pulse reflected from the probe bottom (which is typically in the vicinity of the tank bottom) must necessarily pass through the liquid layer being measured, such measurements based on the time of flight of the pulse reflected from the probe bottom are susceptible to variations in the temperature and dielectric constant of that liquid layer.
In one variation proposed in commonly assigned U.S. nonprovisional patent application Ser. No. 11/650,841 entitled “Scan Lock and Track Fluid Characterization and Level Sensor Apparatus and Method,” a closed-loop scan-lock-track methodology is employed to find and lock onto a surface of interest. Once the surface of interest has been found, samples need not thereafter be taken along the entire transmission line but may instead be taken only in the vicinity of this surface of interest. Targeting the region around the surface of interest in this way permits more efficient use of processing capacity, allowing better spatial resolution to be obtained from a smaller number of samples.
For example, such a surface of interest that could be locked onto and tracked using such scan-lock-track methodology is the air-liquid interface which moves to successively lower heights above the bottom of a fuel tank as fuel within the tank is consumed. In practice, however, it is found that the comparatively weak reflection from the air-liquid interface is not as easy to detect as the strong signal corresponding to the reflection from the probe bottom. Furthermore, liquid-liquid interfaces may be even more difficult to detect than the air-liquid interface. In practice, therefore, where scan-lock-track methodology is used to lock onto and track the reflection from the probe bottom, the perceived location of the probe bottom on an impedance-versus-time trace will depend on the propagation time of the pulse as it travels through liquid layer(s). Conventional scan-lock-track measurements of liquid height calculated based on the arrival time of the pulse reflected from the probe bottom are therefore still susceptible to the effects of temperature and dielectric constant on that propagation time.
Such dependence on temperature and dielectric constant is undesirable in situations where temperature or dielectric constant is unknown (the latter often being the case, for example, when additive, contaminant, or stratified liquids are present), and is particularly unsuited to aircraft fuel gauge applications and other situations where one is generally more interested in knowing mass rather than volume of remaining fuel.
It would therefore be desirable to employ a TDR system that, rather than locking onto and tracking the region in the vicinity of the air-liquid interface and measuring liquid height in a way that is dependent on liquid temperature and/or dielectric constant, instead locks onto and tracks the region corresponding to the entire probe and renders the impedance profile of the various layers in which the TDR probe is immersed with a spatial resolution allowing liquid height to be measured in a way that is comparatively insensitive to changes in liquid temperature and dielectric constant.
For example, it would be desirable to employ a TDR system in which sample collection does not begin simultaneous with transmission of the interrogation pulse but only after first waiting for a time which might be referred to as an offset time. It would furthermore be desirable if this offset time between the time that the interrogation pulse is transmitted and the time that swept sample collection begins (sweep offset) were set so as to cause sample collection to start at or slightly prior to a time when an interrogation pulse edge reflected from the top of the probe would have just arrived at the receiver. Similarly, once sample collection has started, it would be desirable if the size of the increment in the delay between successive swept samples (sweep gain) were set so that, given a reasonable number of samples that can be collected and processed with satisfactory response time, the last of the samples is collected at or slightly after a time when an interrogation pulse edge reflected from the bottom of the probe would have just arrived at the receiver. Doing so would allow sweep offset and gain to be optimized so as to permit maximum resolution of the impedance variations occurring along the length of the probe in correspondence to the available sample processing capacity of the electronics.
However, even if such optimization were to be carried out as described above so that a reasonable number of samples capable of being collected and processed with satisfactory response time were collected at times between the approximate time when an interrogation pulse edge reflected from the top of the probe would have just arrived at the receiver and the approximate time when an interrogation pulse edge reflected from the bottom of the probe would have just arrived at the receiver, there is still the problem of variability in the parameters affecting those reflected pulse arrival times.
For example, even where only a single probe is employed, reflected pulse arrival time will vary depending on such parameters as cable length, probe length, and tank content. In particular, while probe top reflected pulse arrival time might be expected to be relatively constant for a particular cable-probe system, probe bottom reflected pulse arrival time can vary widely depending on tank content due to the effect of dielectric constant on velocity of propagation as noted above. For example, since the dielectric constant of air is lower than that of most liquids, the probe bottom reflected pulse arrival time will typically be later for a tank full of liquid fuel than for an empty tank. Similarly, where variable amounts of water and hydrocarbon fuel are present within a tank, the high dielectric constant of water relative to either hydrocarbon fuel or air can be expected to cause the probe bottom reflected pulse to arrive at the receiver much later when water is present than when water is absent.
Furthermore, in systems where multiple transmission lines contain multiple cables connected to multiple probes in multiple tanks of varying size and content, and these multiple transmission lines are coupled to the same electronics unit, in multiplexed or time-shared fashion for example, the different lengths and characteristics of the respective cables, the different lengths and characteristics of the respective probes, and the different contents of the respective tanks would, without some special adapter or other stratagem, cause probe top reflected pulse arrival time and probe bottom reflected pulse arrival time to vary in accordance with the different cable-probe-tank systems. Furthermore, especially when for reason of convenience or safety there are significant lengths of cable between digital signal processing electronics and probe(s), collection of samples at locations (times) not corresponding to a probe will result in poor response time and/or poor resolution. It would therefore be desirable to be able to conveniently match such electronics to a wide variety of cable-probe-tank systems by varying sweep delay and aliasing in correspondence to cable and probe characteristics as well as tank content. This would make it possible to conveniently adapt the electronics to each cable-probe-tank system, or adapt each cable-probe-tank system to the electronics, so that the train of collected samples covers the range from the approximate probe top to the approximate probe bottom regardless of cable length and other characteristics, regardless of probe length and other characteristics, and regardless of tank content. It would moreover be desirable if this matching of electronics and cable-probe-tank systems could be carried out automatically to seamlessly permit the same or similar electronics to be used with multiple cable-probe-tank systems.
Moreover, even if sweep delay and aliasing were to be optimized for a particular cable-probe-tank system so as to permit maximum resolution of the impedance variations occurring along the length of the probe in correspondence to the available sample processing capacity of the electronics as described above, a technical problem that would remain is the question of what algorithm will permit most accurate determination of the height profile of the various layers in which the TDR probe is immersed within the tank.
It would therefore be desirable to employ an algorithm that would allow such a (space domain) height profile to be accurately derived from the (time domain) impedance-versus-time trace obtained by TDR.
It would furthermore be desirable to employ an algorithm that would allow this height profile to be accurately derived from the impedance-versus-time trace in a way that corrects for the different dielectric constants that the different layers in which the probe is immersed may have.
It would moreover be desirable to employ an algorithm that would allow this height profile to be accurately derived from the impedance-versus-time trace in a way that is insensitive to the dielectric constant of at least one of the layers in which the probe is immersed. That is, it would be desirable to be able to obtain such a height profile even when the dielectric constant of at least one of the layers in which the probe is immersed in unknown. The dielectric constant of such a layer may be unknown because, for example, the chemical composition of the layer is unknown (because of unknown amount of ethanol or other such additive, or water or other such contaminant), or because temperature (and thus density) of that layer is unknown.
That is, even where it is possible to determine dielectric constants of one or more layers based on impedance values for layers derivable from impedance-versus-time traces (impedance axis of TDR trace) and/or based on times of flight through layers derivable from impedance-versus-time traces (time axis of TDR trace), it may be desirable to have an algorithm for calculating layer height that is insensitive to the dielectric constant and/or temperature of at least one layer.
Conversely, even where it is possible to determine layer height in a way that is insensitive to the dielectric constant and/or temperature of at least one layer, it may be desirable to have an algorithm that would permit measurement of dielectric constant of at least one layer. Such an algorithm might be used alone or in conjunction with any of the algorithms referred to above. For example, ability to measure dielectric constant of at least one layer might be desirable because it would make it possible to more accurately convert the time domain profile obtained by TDR to a space domain layer height profile through use of the relationship
  d  ∝      t          ɛ      where d=distance, t=time, and ∈=dielectric constant. As another example, ability to measure dielectric constant of at least one layer might be desirable because it would make it possible to calculate the temperature (and thus density and/or volume) thereof. As yet another example, ability to measure dielectric constant of at least one layer might be desirable because it would make it possible to determine the chemical composition thereof, and thus make it possible to identify such things as contamination, degradation in quality, misfueling (that is, when the tank has inadvertently been filled with the wrong fuel), and/or other phenomena related to chemical composition that manifest as changes in dielectric constant.
Moreover, as alluded to above, the reflection of the interrogation pulse from the bottom of the electrically open- or short-circuited TDR probe has large amplitude. This being the case, the large-amplitude pulse reflected from the probe bottom may swamp or overwhelm pulses reflected from air-liquid or liquid-liquid interfaces, making detection of those interfaces difficult, when those interfaces are close to the probe bottom (which is typically in the vicinity of the tank bottom). For example, a tank that is almost empty or contains only a small amount of fuel will typically have an air-liquid interface that is near the tank bottom. It would therefore be desirable to employ an alternate algorithm that does not depend on detection of reflections from air-liquid or liquid-liquid interfaces when those interfaces are close to the probe bottom.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.